Maximum prospective short circuit current
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The maximum prospective short circuit current is the maximum electrical current which can flow in a particular electrical system under short circuit conditions. It is determined by the voltage and impedance of the supply system. It is of the order of a few thousand amperes for a standard domestic mains electrical installation in the UK, but may be as low as a few milliamperes in a SELV (Safety Extra Low Voltage or, occasionally, Separated Extra Low Voltage) system or as high as hundreds of thousands of amps in large industrial power systems.
[edit] Importance
It is of particular interest when designing an electrical installation because fuses and circuit breakers must be capable of safely breaking the flow of current in the event of a short circuit. When a large electrical current is interrupted an arc may form and if the breaking capacity of a fuse or circuit breaker is exceeded then it may not be able to extinguish this arc meaning that the flow of current will continue, possibly resulting in a fire or explosion.
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In domestic power installations in European countries (230 V 50 Hz AC single phase power) the short circuit current available on the electrical outlets needs to be taken into account when designing electrical power wiring. The short circuit current should not be too high or too low. The effect of too high short circuit current is discussed in the previous paragraph. The short circuit current should be of the order of around 200 A or higher for normal 10 A or 16 A outlet to guarantee that the normal wire protecting fuse or breaker will quickly disconnect the supply in case of short circuit. Quick disconnecting is needed, because in live-ground short circuit the grounding pin potential on the power outlet can rise compared to the local earth (concrete floor, water pipe etc.) during short circuit to a dangerous voltage, which needs to be shut down quickly for safety. If the short circuit current is lower than this figure, special precautions needs to be taken to make sure that the system is safe; those usually include using a residual current device for extra protection. The short circuit current available on the electrical outlets is often measured with a suitable instrument when examining electrical installations to make sure that the short circuit current is within reasonable limits. Having a high enough short circuit current on the outlet will also tell that the resistance from the electrical panel to the outlet is reasonably low, so there will not be too high voltage losses on the wires when outlets are loaded normally.
